1. Field of the invention
The present invention relates, in general, to a novel method for producing films with an aliphatic copolyester and, more particularly, to an improvement in the draw ratio of the films along with the method.
2. Description of the Prior Art
Aromatic polyesters, especially polyethylene terephthalate, are superior in physical and chemical properties and are widely used to make various applications, such as films, bottles, glass fiber-reinforced plastics, adhesives, paints and the like.
In contrast, aliphatic polyesters, although used in the forms of paint or adhesive, lacks of processability, especially moldability to film. In addition, it is difficult to derive desired melt viscosity and melt strength therefrom. Furthermore, the films of aliphatic polyester, if formed, are poor in mechanical physical properties. That is to say, when aliphatic compounds are employed as the dicarboxylic acid and glycol components for polyester, the polyester, even if obtained by polycondensation after esterification or ester-interchange, is not endowed with enough melt viscosity or melt strength to be formed or processed into a film. Although it is formed and processed into a film, the final product is very inferior in workability and mechanical physical properties.
In order to improve the mechanical physical properties and melt strength, mainly used is a method of increasing the molecular weight thereof. In this method, however, it is of importance to appropriately control the increase of the molecular weight because too large molecular weight of the polyester may cause to raise the melt viscosity overly, which in turn results in difficulty in passing through an extruder on processing.
With regard to the melt strength of a polymer, it chiefly depends on the elasticity of a polymer melt. Exemplary factors affecting the elasticity include the distribution of molecular weight, the extent of branch, the kind and amount of additives. As the molecular weight is large, as the extent of branch is high and as the entanglement of polymer melt is serious, the elasticity becomes enhanced. In case of polyester, full well is it known that stearic acid, or talc or silica particle brings the melt strength into effect.
In addition, crystallization rate is another factor having great influence on the moldability of a polymer. The crystallization rate and crystallinity of a polymer can be controlled according to a processing method and thus, it also is of importance.
Japanese Patent Laid-Open Publication Nos. Heisei 4-189822 and 4-189823 disclose methods for producing polymers with enhanced melt viscosity and melt strength in which aliphatic dicarboxylic acids and aliphatic divalent glycols are subjected to esterification and polycondensation and then added with isocyanate compounds, to increase the molecular weights of the polymers. However, these methods are uneconomical because additional isocyanate compound are used. Besides, the separate addition process causes complexity and difficulty in operating the method.